I'm managing two clusters, one of which runs CentOS 6 while the other runs CentOS 7. With a packages.yaml like:

packages:
  flex:
    paths:
      flex@2.5.37 arch=linux-centos7-x86_64: /usr
      flex@2.5.35 arch=linux-centos6-x86_64: /usr
    buildable: False

things work great on CentOS 7. But on CentOS 6, I'm seeing:

$ spack spec flex
...
==> Error: The spec 'flex' is configured as not buildable, and no matching external installs were found

If I had to guess, I bet it's choosing flex@2.5.37 because it's newer, and then getting an error because arch=linux-centos7-x86_64 says it isn't for CentOS 6.